Restyle the Slides

## Do you have an eye for design?
Help us improve the look of the slide deck.
If you need help, check out the Appendix in the training manual.

Start by reviewing the slide deck and the Appendix in the training manual. Done means the slides have a consistently improved visual style, but the issue does not name files or define specific acceptance criteria.

索引モデルが issue の本文から書いたものです。